A Customized Way to Calculate Heating Needs

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Dickinson LPG fireplace

Although rules of thumb for heating requirements exist, every boat is different. A simple way to measure the heating requirements of a boat is to use a space heater and a few simple calculations. A typical electric space heater is 1,500 watts, or 5,150 British Thermal Units per hour output when set on high. Pick a cool day (does not need to be the coldest expected day) and observe how many watts it takes to warm the boat and how many watts are required to hold that temperature. Several hours should be allowed for this test (it takes time for the furnishings and hull liner to warm up), and it is best done at night when temperatures are cooler.

To figure required heater output in BTUs, use this formula:

Required BTUs = watts x 3.41 x (lowest anticipated exterior temperature – desired interior temperature)/(exterior temperature during test – test interior temperature during test)

In the case of our test boat, we determined we needed 1.5 x 1,500-watt heaters (6,394 BTU/hour) to maintain approximately 75 degrees in the cabin on a 20-degree night (our projected coldest cruising temperature). Unfortunately, the P9000 delivers only 3,872 BTU/hour (at 61 percent of maximum load), and the space available would not fit a larger heater. On nights when temperatures dip below freezing, we have to bundle up, or close off all unoccupied cabin spaces.

Does the Dickinsion P9000 actually produce the heat it claims? While direct measurement of output is complicated, engineers have a short-cut method used to estimate the efficiency of boilers and heaters. It is based on the principle of conservation of energy. If we know the heating value of the fuel and we can measure the energy leaving through the stack, whatever remains must have been delivered as cabin heat.

After measuring the stack temperature (329 degrees), ambient temperature (20 degrees), excess oxygen in the stack (6 percent), and pre-heat temperature, we calculated an overall efficiency of 88 percent; quite good for a tiny heater and comparable to modern home heating furnaces. The stated heat output of the P9000 (3,872 BTU/hour) roughly matches our observations.

Darrell Nicholson, editor of Practical Sailor, grew up boating on Miami’s Biscayne Bay on everything from prams to Morgan ketches. Two years out of Emory University, after a brief stint as a sportswriter, he set out from Miami aboard a 60-year-old wooden William Atkin ketch named Tosca. For 10 years, he and writer-photographer Theresa Gibbons explored the Caribbean, crossed the Pacific, and cruised Southeast Asia aboard Tosca, working along the way as journalists and documenting their adventures for various travel and sailing publications, including Cruising World, Sail, Sailing, Cruising Helmsman, and Sailing World. Upon his return to land life, Darrell became the associate editor, then senior editor at Cruising World magazine, where he worked for five years. Before taking on the editor’s position at Practical Sailor, Darrell was the editor of Offshore magazine, a boating-lifestyle magazine serving the New England area. Darrell has won multiple awards from Boating Writer’s International, including the Monk Farnham award for editorial excellence. He holds a U.S. Coast Guard 100-ton Master license and has worked as a harbor pilot and skippered a variety of commercial charter boats.

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